1. Field of the Invention
The present invention relates to a structure for coupling between a low temperature circuitry and a room temperature circuitry, and particularly to a joint structure for coupling between a low temperature circuitry such as a superconducting circuit and a room temperature circuitry.
2. Description of Related Art
Devices which utilize superconducting phenomena operate rapidly with low power consumption so that they have higher performance than conventional semiconductor devices. Particularly, by using an oxide superconducting material which has been recently advanced in study, it is possible to produce a superconducting device which operates at relatively high temperature, such as higher than a liquid nitrogen temperature. Researches on Josephson junction devices, superconducting transistors, superconducting field effect devices, etc. utilizing those oxide superconductors are now in progress.
On the other hand, it has been tried to utilize lower operating temperature for semiconductor devices so as to improve their operating speeds and to be free from thermal noises. By this, the semiconductor devices can operate with reduced current. Cooling system is also effective to take stably away dissipated heat at each device, which is essential for high degree of integration and rapid operation.
Some extended researches have tried to use semiconductor devices combined with superconducting devices at a liquid nitrogen temperature.
Although the superconducting device utilizing the oxide superconductor material (high T.sub.c copper oxide superconductor) can operate at an extremely higher temperature than that utilizing a metal superconductor, cooling system is required to keep them at least liquid nitrogen. Therefore, the operating temperature of the superconducting device utilizing the oxide superconductor is lower around 200.degree. C. or more than the room temperature.
In a prior art, the superconducting device and the cooled-down semiconductor device are connected to a room temperature device by a conventional method, such as bonding wires, bonding pads and connectors, or in some cases using probing pins, etc. In this case, although the superconducting device and the cooled-down semiconductor device are maintained at a liquid nitrogen temperature, but the bonding wires, connectors and probing pins are at higher temperature than that. Therefore, the bonding wires, connectors and probing pins are inherently subjected to large temperature gradient.
The bonding wires and probing pins have small cross-sectional areas so as to have large thermal resistance. This large thermal resistance inevitably gives rise to thermal stress to the bonding wires and probing pins so that they are prone to fracture by means of heat cycles between the low temperature and the room temperature.
In case of a lot of input and output lines and of applying a high frequency signal, heat transfer from the bonding pads is not negligible so that temperatures of the superconducting device and the cooled-down semiconductor device may be elevated unexpectedly. This may hurt device characteristics or may increase operating costs of the devices for additional cooling. In addition, this may also cause partial transition to normal state (quenching) which makes the superconducting device out of order or sometimes destroys it.